1. Field of the Invention
This invention relates to an expansion fastener consisting of a female member and a male member for fastening together two metal or plastic plates or sheets in an overlapped state. More particularly, the invention relates to a fastener of the type noted, in which the male and female members are broken apart at an intermediate position of their length to release the overlapped state of the two plates when one thereof experiences an external impact force.
2. Prior Art Statement
An expansion fastener, which comprises a female member including a cylindrical portion, a flange portion extending radially outwardly from the upper end of the cylindrical portion and a plurality of elastic leg portions defined by radially spaced slits and each having a radially inward projection projecting from the lower end of the cylindrical portion, and a male member including a shaft portion passing through the cylindrical portion of the female member, a head portion extending radially outwardly from the upper end of the shaft portion and an enlarged diameter portion extending radially outwardly from lower portion of the shaft portion and functioning, when the head portion is substantially aligned with the upper surface of the flange portion of the female member, to engage the projections of the lower ends of elastic leg portions of the female member to cause radially outward deformation of the elastic leg portions, is well known in the art (as disclosed in, for instance, Japanese Utility Model Publication No. 61-166212).
When an impact force acts on one of two plates fastened together in an overlapped state by this fastener, the relative movement of the plates is resisted by the fastener, thus causing deformation or rupture of the plates. In other words, the fastener gives rise to various troubles depending on the deformation of the plates.
For example, with reference to FIG. 1, when a bumper face 3 and a reinforcement 4 of an upper portion of an automobile front bumper 2 are secured together by a prior art fastener 1, at the time of impact the bumper face 3 strikes the lower end of a body panel 5 and causes deformation thereof as shown by phantom lines in FIG. 1. In this case, part of the body is broken. Therefore, i is necessary to replace the front bumper 2 and repair the body panel 5, which results in high repair cost. In the Figure, reference numeral 6 designates a urethane member and numeral 7 a front grille.